I don't quite understand question 16

@Marianne-Xiong, Happy to help. Let's take a look at (D) and (B). (D) says, "The reflective properties of the material of which comets are composed vary considerably from comet to comet." The problem with (D) is that we don't have any sort of idea of how reflective the other comets are. We could have simply just been incorrect about all comets, so this is why (D) is out. (B) says, "Previous estimates of the mass of Halley’s comet which were based on its brightness were too low." (B) looks great. Halley's comet has greater mass than we would have anticipated based on its brightness, so the former estimates of Halley's mass had to have been too low, making (B) the correct answer. Hope this helps. Let us know if you have any other questions!

Why not C?

Shouldn't answer (B) indicate the opposite?

@ Danielle. I too originally thought C. But I see why B is correct. There is no indication that they got the brightness wrong. Rather the brightness which they saw in their mind reflected a certain mass which they then discovered that its mass reflects much less than they thought. Which would mean that for = brightness there would need to be greater mass than originally thought which is what answer B says, that original estimates of mass were too low.

Dace its pretty tough to keep track of all the effects, but I like to walk my way through the story. If a comet is brighter, then we think it has more mass. However, if a comet is artificially dark, as is the case here, this means that we have underestimated its mass. Since it appears less bright than it should, we would thus think that it is less massive than it really is.